Hi tmuikku,Another simplfied textual example then5kHz breakup resonance. The breakup is there on the cone as physical property. Its 6db peak on manufacturer datasheet on otherwise nice frequency response. We plan to crossover below the breakup.
We have 16th order lowpass filter in DSP at 1.7kHz, not much of the sound gets to amplifier or to driver at 5kHz anymore, nice. But, full sound goes to amp and driver at 1.7kHz, which the driver motor adds harmonics to due to non-linearities in the motor and 3rd order harmonic now lands on the 5kHz and excites the resonance and gets extra 6db boost from it, comes out louder than without the resonance, or other harmonics around the resonance. The 5Khz sound did not come from source, or from the amp, but was generated in process in the driver.
Now, the original sound we feed to the amp and to driver is heard only up to 1.7Khz where steep low pass has cut it out, but all the harmonics generated in the driver (on its passband) emit all the way up to many kHz, unless are filtered out as well. Acoustic low pass or passive comoonents reducing current in the driver above crossover.
that's what I'd like to see....if the driver motor itself produces significant 3rd harmonics with a 1.7kHz fundamental.....(in the absence of any drive signal to the driver at 5kHz). back to wanting to see a single 1.7kHz sine THD test.
If there is measured 3rd harmonic THD at the single 1.7kHz sine, that gets significantly reduced by adding the passive notches, then I'm convinced.
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The way I understand it, the third harmonic of 5/3 khz is at 5 Khz where the driver resonance/peak occurs. if the driver were flat it would be down 40 or 50 db and we wouldn't think twice about it. Since the driver peak brings it up 20 db at 5 khz, its only down 20 or 30 db. A low slope passive crossover only pushes it down another 12 db to -32 to -42 db which is marginal In that scenario, the difference between the series vs parallel notch is significant. with DSP, we can push it down out of sight. with only passives, we have to be clever. I think it was wise that we chose to go active long ago.
The question I have is whether the high Z from the from the notch filter suppresses the nonlinearity near 5/3 khz in the first place. If that were the case, then I might be temped to do a hybrid crossover. If its just another way of filtering then its only the end result that matters.
The design I have on the drawing board crosses over to a CD/tweeter near 1 Khz so that 5/3 khz fundamental is outside my midwoofers passband but not very far down its low pass filter's slope. I would put a -20 db IIR PEQd on 5/3 khz amd 5 khz in addition to the FIR auto-eq flattening then application of textbook low pass filter.
The question I have is whether the high Z from the from the notch filter suppresses the nonlinearity near 5/3 khz in the first place. If that were the case, then I might be temped to do a hybrid crossover. If its just another way of filtering then its only the end result that matters.
The design I have on the drawing board crosses over to a CD/tweeter near 1 Khz so that 5/3 khz fundamental is outside my midwoofers passband but not very far down its low pass filter's slope. I would put a -20 db IIR PEQd on 5/3 khz amd 5 khz in addition to the FIR auto-eq flattening then application of textbook low pass filter.
Hi, sorry forgot to say hi on last post
Yes, and if you can do IMD as well it would be cool since the distortion in driver motor worsens with excursion. I've got no idea how audible this is and thats the ultimate test, there is of course no point complicate things if its not audible.4 way MEH system, not sure there is any point to it... and direct radiating two way would benefit most as the woofer does some serious excursion in the application.
On a MEH this "out of band" distortion are also point source, perhaps masked fine by tweeter for example. Take traditional speaker and the distortion of woofer comes from different location (than tweeter) perhaps adding to audibility.
MEH is such a good concept in so many ways
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Well, if its a problem I think you could see in RTA that slope of 96db/octave is something less? If there is distortion products -40db down they should make the slope appear much shallower I think. Just pink noise to some woofer you have around, apply 16th order low pass and look RTA if it really is 96db down octave above, or much less. Then, if you have some spare inductor somewhere hook it in series and see if the slope gets steeper.
Hi, check out the purifi paper, THD really goes down. But then again THD does not correlate to perceived quality so, we need to test it in our applications to believe
Yeah its easy to deal with the direct sound, but distortion originating at the driver is not affected no matter how much we suppress in DSP, we need good drivers and limit current in the driver to cut down the distortion. Cone breakup happens still for what ever current is left there flowing through voice coil at the breakup frequency.
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what you describe is exactly what happens: 1.7k sine results in 5k harmonic back emf in the motor which gets converted to current depending on the driver circuit impedance (the impedance the driver sees). If it is a regular amp with low output impedance then then the conversion factor is 1/Re. if you then insert the notch that provides high impedance at 5k then the current is significantly reduced which results in a suppressed 5k 3rd harmonic and the distortion at 1.7k will be reduced. The distortion reduction requires that the speaker load impedance is changed and this is not possible with just an active filter and ordinary amp irrespective of the filter order (of course if the filter cuts below 1.7k then the distortion peaks go away )
If you attenuate 5 khz, the distortion at 1.67 Khz will be reduced because the signal at 5 khz IS part of the distortion at 1.67 khz. There is more of the distortion of 1.67 khz distortion at 10 and 15 khz. Its counter intuitive but that is simply the definition of harmonic distortion.
I now see that the parallel trap isn't just a filter - it also reduces the conversion of back emf into harmonic current which the driver then turns into sound.
I now see that the parallel trap isn't just a filter - it also reduces the conversion of back emf into harmonic current which the driver then turns into sound.
Thanks for that !
i guess i'm still most interested in, is the passive notch step a worthwhile addition to the steep linear phase xovers I use.
I'm 100% sold on those xovers....it's just a question to me if there is more SQ on the table from another step like the passive notch.
If you ever test your driver with a steep linear phase low pass, I'd love to see the results....especially both with and without the passive notches in place.
The passive notches look like more work than I'd like to be involved with, but if clear advantages are there, i could see doing such. Thx.